Branched polyethylene can be made in a multi-step process by contacting ethylene, a catalyst that can oligomerize ethylene to one or more alpha-olefins, and a polymerization catalyst which can copolymerize ethylene and xcex1-olefins, in a first step (such as in a continuous stirred tank reactor), then continuing the reaction in a subsequent step (such as in a plug flow reactor) in the substantial absence of added ethylene.
Polyolefins are useful items of commerce, being made in immense annual quantities and used for a variety of purpose, such as molding resins, films, elastomers, and other items. Polyolefins are most often prepared by polymerization processes in which a transition metal containing catalyst system is used. Depending on the process conditions used and the catalyst system chosen, polymers, even those made from the same monomer(s) may have varying properties. Some of the properties which may change are molecular weight and molecular weight distribution, crystallinity, melting point, branching, and glass transition temperature. Except for molecular weight and molecular weight distribution, branching can affect all the other properties mentioned.
Various reports of xe2x80x9csimultaneousxe2x80x9d oligomerization and polymerization of ethylene to form (in most cases) branched polyethylenes have appeared in the literature, see for instance WO90/15085, WO99/50318 (corresponding to U.S. application. Ser. No. 09/619,509, filed Jul. 19, 2000, now abandoned, which is a continuation of U.S. application. Ser. No. 09/273,409, filed Mar. 22, 1999), now U.S. Pat. Nos. 6,214,761, 5,753,785, 5,856,610, 5,686,542, 5,137,994 and 5,071,927; C. Denger, et al, Makromol. Chem. Rapid Commun., vol. 12, p. 697-701 (1991); and E. A. Benham, et al., Polymer Engineering and Science, vol. 28, p. 1469-1472 (1988). All of the above are incorporated by reference herein for all purposes as if fully set forth.
In most of these systems one or more xcex1-olefins is produced by catalysis with an ethylene oligomerization catalyst, and then a copolymer of the xcex1-olefin(s) and ethylene is produced by (another) polymerization catalyst. The result is normally a branched polyethylene made from ethylene alone (although other olefins such as xcex1-olefins may also be added if desired).
In most instances, the rate of formation of polymer is faster than the rate of oligomerization, so that the incorporation of xcex1-olefin(s) is often quite low, and of course the resulting branching is quite low. When the rate of xcex1-olefin production is comparable to the rate of polymer production it is possible to make relatively more xcex1-olefin, and consequently to obtain a higher branching level in the polymer, see for instance previously incorporated World Patent Application 99/50318 (corresponding to U.S. application Ser. No. 09/619,509, filed Jul. 19, 2000, now abandoned, which is a continuation of U.S. application. Ser. No. 09/273,409, filed Mar. 22, 1999now U.S. Pat. No. 6,214,761). However this can lead to another problem, namely, the presence of residual xcex1-olefin(s) in the resulting polymer product.
While this may not be too serious if the xcex1-olefin(s) are relatively volatile (say containing 8 or fewer carbon atoms), when less volatile xcex1-olefin(s) are present, their removal may be come difficult, and their presence in the polymer deleterious from a flammability, toxicity or end use (e.g., in contact with food, as food wrapping) viewpoint. Thus a production method that minimizes or even substantially eliminates the presence of these xcex1-olefin(s) in the final product is desirable.
This invention concerns a process for the production of a branched polyethylene comprising the steps of contacting, under polymerization conditions:
(a) ethylene;
(b) an oligomerization catalyst that oligomerizes ethylene to one or more xcex1-olefins of the formula R20CHxe2x95x90CH2, wherein R20 is n-alkyl containing an even number of carbon atoms, and
(c) a transition metal polymerization catalyst that co-polymerizes ethylene and one or more of the xcex1-olefins produced by the oligomerization catalyst, to produce a first (intermediate) product comprising a predominant amount of a branched polyethylene, a first residual xcex1-olefin content and, optionally, an amount of unreacted ethylene; then
(2) allowing the first product to further react under conditions to (co)polymerize at least a portion of the first residual xcex1-olefin content, in the substantial absence of added ethylene, to result in a second (end) product comprising a branched polyolefin having a second residual xcex1-olefin content, provided that said second residual xcex1-olefin content is at least 20% less than said first residual xcex1-olefin content.
This invention also concerns a process for the production of a branched polyethylene, comprising the step of contacting, under polymerizing conditions:
(a) ethylene,
(b) an ethylene oligomerization catalyst which is capable under the process conditions of producing one or more olefins of the formula R20CHxe2x95x90CH2 wherein R20 is n-alkyl containing an even number of carbon atoms, and
(c) a polymerization catalyst which contains one or more transition metals and which is capable under process conditions of copolymerizing ethylene and one or more olefins of the formula R21CHxe2x95x90CH2, wherein R21 is n-alkyl, provided that a major portion of the branched polyethylene is produced first in a continuous stirred tank reactor (CSTR) or its equivalent, and that a minor portion of said branched polyethylene is produced subsequently in a plug flow reactor (PFR) or its equivalent.